/**
 * JDBM LICENSE v1.00
 *
 * Redistribution and use of this software and associated documentation
 * ("Software"), with or without modification, are permitted provided
 * that the following conditions are met:
 *
 * 1. Redistributions of source code must retain copyright
 *    statements and notices.  Redistributions must also contain a
 *    copy of this document.
 *
 * 2. Redistributions in binary form must reproduce the
 *    above copyright notice, this list of conditions and the
 *    following disclaimer in the documentation and/or other
 *    materials provided with the distribution.
 *
 * 3. The name "JDBM" must not be used to endorse or promote
 *    products derived from this Software without prior written
 *    permission of Cees de Groot.  For written permission,
 *    please contact cg@cdegroot.com.
 *
 * 4. Products derived from this Software may not be called "JDBM"
 *    nor may "JDBM" appear in their names without prior written
 *    permission of Cees de Groot. 
 *
 * 5. Due credit should be given to the JDBM Project
 *    (http://jdbm.sourceforge.net/).
 *
 * THIS SOFTWARE IS PROVIDED BY THE JDBM PROJECT AND CONTRIBUTORS
 * ``AS IS'' AND ANY EXPRESSED OR IMPLIED WARRANTIES, INCLUDING, BUT
 * NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND
 * FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL
 * CEES DE GROOT OR ANY CONTRIBUTORS BE LIABLE FOR ANY DIRECT,
 * INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
 * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
 * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
 * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED
 * OF THE POSSIBILITY OF SUCH DAMAGE.
 *
 * Copyright 2000 (C) Cees de Groot. All Rights Reserved.
 * Contributions are Copyright (C) 2000 by their associated contributors.
 *
 * $Id: TranslationPage.java,v 1.1 2000/05/06 00:00:31 boisvert Exp $
 */

package com.sunchangming.stmdb.recman;

/**
 * Class describing a page that holds translations from physical rowids to
 * logical rowids. In fact, the page just holds physical rowids - the page's
 * block is the block for the logical rowid, the offset serve as offset for the
 * rowids.
 */
final class TranslationPage extends PageHeader {
	// offsets
	static final short O_TRANS = PageHeader.SIZE; // short count
	static final short ELEMS_PER_PAGE = (RecordFile.BLOCK_SIZE - O_TRANS) / PhysicalRowId.SIZE;

	// slots we returned.
	final PhysicalRowId[] slots = new PhysicalRowId[ELEMS_PER_PAGE];

	/**
	 * Constructs a data page view from the indicated block.
	 */
	TranslationPage(BlockIo block) {
		super(block);
	}

	/**
	 * Factory method to create or return a data page for the indicated block.
	 */
	static TranslationPage getTranslationPageView(BlockIo block) {
		final BlockView view = block.getView();
		if (view != null && view instanceof TranslationPage)
			return (TranslationPage) view;
		else
			return new TranslationPage(block);
	}

	/** Returns the value of the indicated rowid on the page */
	PhysicalRowId get(int offset) {
		final int slot = (offset - O_TRANS) / PhysicalRowId.SIZE;
		if (slots[slot] == null)
			slots[slot] = new PhysicalRowId(block, offset);
		return slots[slot];
	}
}
